Having had to navigate Atlanta traffic more times I care to remember, I would second (or third) the advice about trying to get past the city before stopping for the night. Partly because of those issues, whenever we drive we take the "other" route, through Buffalo, which uses 95 to get into Florida. Faster and more scenic - from Toronto. But from Sarnia that's way out of the way.
We've had dozens of road trips all over the States and based on that I think you're right in not having to worry too much about getting a highway-side room on a Sunday night. And because of so many variables, most importantly simply seeing how long I feel like driving, we almost have never reserved a room in advance. What we still do (as recently as last August) is when we hit the State in which we think we'll stop, we grab copies of those red and green hotel coupon books that against all odds are still widely available in many travel plazas, welcome centres and truck stops. Not for the coupons per se, but to see the list of properties that are down the road, and what they offer, where they're located, etc.
It's kind of a family game. I drive, my wife - or daughter, who's now 15 - will read off the descriptions of upcoming hotels, and their features (free continental breakfast is a must). Once we settle on the city/area we target for our stop, my wife calls our pick(s) just to ensure they have a vacancy.
We've never not found a room this way, although it can be a bit of a gamble on holiday weekends, or if there events nearby (we learned the hard way to avoid places where a NASCAR race is happening - more than once). But it's a fun tradition.
In any event - have a blast! I'm jealous - we absolutely love love road trips. Our newest tradition is we draw names from a hat (although there's only the three of us) and then when we stop at one of those giant Pilot/Flying Js/Love truck complexes for gas or snacks we each secretly buy the most outrageous/garish/hilarious t shirt for whoever we picked, and they have to wear that on our first day driving back. Kinda takes the sting out of that day on the way home.
But we're flying this time - also last week of August (such a great time to go). But now I wish we were driving ...
